1. Field of the Invention
The present invention relates to a method for manufacturing an optical element called a microlouver which restricts the range of transmitted light in the direction of emission. The present invention also relates to a display apparatus such as liquid-crystal display (LCD), a plasma display, an illuminating optical apparatus and electronic apparatus that have an optical element manufactured by the optical element manufacturing method.
2. Description of the Related Art
Liquid-crystal displays are used as displays for various information processing devices such as mobile phones, personal digital assistants (PDAs), automatic teller machines (ATMs), and personal computers. Liquid-crystal displays with wide viewing angles have been put into practical use in recent years. With the increase in the size and versatility of liquid-crystal displays, there is a growing demand for liquid-crystal displays having various light distribution characteristics. In particular, there are growing demands to restrict viewing angles so that the display is not visible to other persons to prevent information leakage and to prevent light from being emitted in unwanted directions.
To meet these demands, displays equipped with a microlouver to restrict the viewing angle (or light emission range) have been proposed and some of them have been brought into practical use.
When a viewer views a large display from directly in front of around the center of the display, the center portion of the large display is bright and an image displayed in the portion is visible to the viewer. On the other hand, the regions near the edges of the large display are dark and the image displayed in those regions is obscured.
As a result, the viewability of the entire image displayed on the large display to the viewer can be degraded. This is because, as shown in FIG. 1., the light absorbing layer of the microlouver 201 provided in front of display 200 is formed substantially perpendicular to the surface of display 200
In-vehicle displays require customized light distribution characteristics. In-vehicle displays are used as navigation monitors or television monitors. Such in-vehicle displays are often located in the center portion of a dashboard. If the viewing angle of the upper side of the display is wide, an image on the display is reflected in the windshield, which can interfere with driving of the vehicle. Therefore, the viewing angle in the vertical directions must be limited. Navigation information displayed is primarily required by the driver and a passenger located in the passenger seat. Therefore, the in-vehicle navigation display requires a microlouver that can distribute light to the driver and passenger seats.
On the other hand, a television monitor should be easily visible to passengers other than the driver. Therefore, the in-vehicle display of a television monitor requires microlouver 301 that distributes light to the passenger located in passenger seat 310c and to passengers in rear seat 310b and that does not distributes light to driver 310a. U.S. Pat. No. 3,919,559 discloses a method for manufacturing a microlouver having a predetermined light distribution characteristic. First, a member that consists of a transparent resin layer and an opaque light absorbing layer arranged at the same angle are provided and then curved into an arch. Then, pressure is applied to the curved member to flatten the member again. As a result of these steps, a microlouver having a light distribution characteristic that converges light to a predetermined position can be provided.
However, the manufacturing method disclosed in U.S. Pat. No. 3,919,559 which applies heat and pressure to a temporarily curved member to flatten it again to slant the light absorbing layer produces a density distribution in the transparent resin layer portions during the stage at which the light absorbing layer is slanted. That is, the density of the transparent resin layer portions increases in a region where the distance between adjacent light absorbing layer portions is reduced by the slant of the light absorbing layer portions. On the other hand, the density of the transparent resin layer portions decreases in the region where the gap between adjacent light absorbing layers portions is wide. That is, the manufacturing method disclosed in U.S. Pat. No. 3,919,559 requires consideration of the effect of the density of the transparent resin layer portions when customizing light distribution characteristics. In addition, the manufacturing method disclosed in U.S. Pat. No. 3,919,559 mechanically transforms the member under heat and pressure. Therefore, microlouvers can be damaged during manufacturing, resulting in low yields. Furthermore, the mechanical transformation of the members under heat and pressure can curve the interface surface between the transparent layer and the light absorbing layer of the microlouver and therefore it can become difficult to ensure flatness of the interface surface.